This invention relates to friction materials and their manufacture, and more particularly to brake pads.
Brake pads are usually manufactured by one of two well-known techniques. In the first method, known as press-forming, an uncured pad containing a binder, reinforcing fibers, fillers and friction modifiers is prepared with substantially finished density using the action of pressure in a suitably shaped die. The binder, normally a thermosetting resin or vulcanizable elastomer or mixtures thereof, flows under the high pressure in the die to contact the reinforcing fibers and particles of filler or friction modifier but is not substantially cured in the die. Curing of the binder is achieved by subsequent baking in an oven. In the second method, known as press-curing, a mixture of fibers, binders, fillers, and friction modifiers is charged to a mold cavity in a die and then molded under heat and pressure sufficient to substantially cure the binder. The mix may be charged to the die as a loose particulate mix or this mix may be molded under pressure only into the form of a preform first. The pads in this case are also usually baked at temperatures of 150 degrees C.-250 degrees C. to complete the cure of the binder and also to improve friction properties.
There have been proposals to replace asbestos fibers in friction materials by other fibers such as cellulose, glass, mineral wools (e.g., basalt or slagwools), alumino-silicate, polyacrylonitrile, polyamides, aromatic polyamides and carbon fibers, UK Pat. Nos. 1604827, 1604828 and 1604839 being examples of such proposals. We have found that conventionally manufactured brake pads using non-asbestos fibers are difficult to make with anything other than low volumes of fibers e.g. up to 20 percent by volume. They are also considered to achieve densities which are close to the theoretical densities of the mix of materials used. This results in impermeable structures with very low void volumes. Such non-asbestos brake pads are hard and dense and often possess good performance but can have poor resistance to cracking at high temperatures.
We have found that non-asbestos brake pads may be manufactured with higher volumes of fiber with all its attendant advantages, and that the structural integrity and fade properties of the brake pads may be remarkably improved thereby.